codegen: an `array[N] ptr array[M] T` argument is declared as `T* a_p)[M]`

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Valutazione

Difficoltà
4/5
Tempo stimato
1-2 giorni
Idoneità per principianti
57/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
c
Ambito
compilers

Direzione di ricerca

Read internal/cgen/expr.go around lines 611–620 and 649–678, where the issue identifies the type handling and declarator dimensions. Use the provided Wuffs repro to generate C, then check that the result is a valid declarator such as uint8_t (*a_p[2])[3] or that generation reports an error. Confirm the generated C compiles with a C compiler.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Hi,

General context of this work is in #188, including the AI-disclaimers.

No in-tree codec is affected by this issue; no std function has an argument that is an array of pointers to arrays. Claude judges the prospective risk of it as "low": the generated C does not compile, and the related nestings fail generation.

The following is Sol 6.1's writeup of the issue. Includes a repro, the generated declarator, compile errors from gcc 13.3, clang 18, MSVC and clang-cl 18, a control that compiles, and generation errors for two other nestings. The cause is reasoned from source.

For an array[2] ptr array[3] base.u8 argument, cgen emits uint8_t* a_p)[3], with an unmatched parenthesis and no outer array dimension. writeCTypeName strips the outer arrays before recognizing the pointer to an array, but writes the closing parenthesis and dimensions from the full type.

Minimal repro:

pub struct repro?(
    buf : array[4] base.u8,
)

pub func repro.f!(p: array[2] ptr array[3] base.u8) {
    args.p[0][1] = 7
}

Actual: at b36a81d3, wuffs-c gen -package_name repro arr_ptr_arr.wuffs exits 0 with empty stderr. The prototype is:

wuffs_repro__repro__f(
    wuffs_repro__repro* self,
    uint8_t* a_p)[3]);

Compiling the amalgamation with WUFFS_IMPLEMENTATION defined fails with gcc 13.3 (-std=c99 -c, exit 1), clang 18 (exit 1), MSVC (cl /c, exit 2) and clang-cl 18 (exit 1):

arr_ptr_arr-release.c:7939:1: error: ‘wuffs_repro__repro__f’ declared as function returning an array
arr_ptr_arr-release.c:7941:21: error: expected declaration specifiers before ‘)’ token
arr_ptr_arr-release.c:7941:21: error: expected function body after function declarator
arr_ptr_arr-release.c(7941): error C2059: syntax error: ')'
arr_ptr_arr-release.c(7939): error C2090: function returns array
arr_ptr_arr-release.c(7941,21): error: expected function body after function declarator

Control: a ptr array[3] base.u8 argument, without the outer array, is emitted as uint8_t* a_p and compiles. Two other nestings fail generation: ptr array[2] ptr base.u8 and ptr ptr array[3] base.u8 arguments produce cannot convert Wuffs type "ptr array[2] ptr base.u8" to C and cannot convert Wuffs type "ptr ptr array[3] base.u8" to C.

Expected: either a valid declarator for an array of pointers to arrays, such as uint8_t (*a_p[2])[3], or a generation error.

Cause: the outer arrays are stripped into x, and the pointer-to-array case is recognized on x:

https://github.com/google/wuffs/blob/b36a81d3352604fee1fee3661f39909706b1b966/internal/cgen/expr.go#L611-L620

The closing parenthesis and dimensions are then written from n, the full type. The outer array shifts what x.Inner().Inner() refers to:

https://github.com/google/wuffs/blob/b36a81d3352604fee1fee3661f39909706b1b966/internal/cgen/expr.go#L649-L678

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